JP2832334B2 - Thermoelectric conversion performance evaluation method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for evaluating the performance of a thermoelectric conversion element used in thermoelectric conversion technology.

[0002]

2. Description of the Related Art Conventionally, thermoelectric cooling and thermoelectric generation using thermoelectric conversion elements (hereinafter simply referred to as thermoelectric elements) have been proposed. In the thermoelectric conversion technology, in order to improve the conversion efficiency of the thermoelectric cooling and the thermoelectric power generation, it is advantageous to use a high-temperature heat source as much as possible to increase the temperature difference. However, thermoelectric materials made of homogeneous materials used in conventional thermoelectric conversion technology have their thermoelectric performance dependent on temperature,
Since the conversion efficiency has a peak in a specific temperature range, there is a limit to improving the conversion efficiency over the entire temperature range.

Therefore, in recent years, the carrier concentration in the thermoelectric element has been changed from one end to the other end of the thermoelectric element, and the carrier concentration has been optimally gradient-distributed so as to obtain the maximum thermoelectric conversion efficiency. Development of concentration gradient thermoelectric elements and composite of multiple thermoelectric element materials with the highest figure of merit in each temperature range using a gradient structure to achieve high thermoelectric conversion efficiency over a wide temperature range from low to high temperatures Development of an inclined structure composite thermoelectric element is underway.

When evaluating the thermoelectric conversion characteristics of a thermoelectric material,
Mainly, the values of the electric resistance (σ), thermoelectromotive force (α), and thermal conductivity (κ) of the thermoelectric material are measured, and the thermoelectric conversion performance index (Z) defined by Z = σα ² / κ is determined. Performance evaluations are being conducted. In a conventional homogeneous material, as shown in FIG.
Since the electric resistance, the thermoelectromotive force, and the thermal conductivity are all functions of temperature, there is a peak in the performance at a specific temperature, and the evaluation can be performed at each temperature.

[0005]

However, in the carrier concentration gradient thermoelectric element and the gradient structure composite thermoelectric element described above, since different parts inside the material have different temperature functions, the broken line a in FIG. As shown by b and c, the maximum thermoelectric conversion performance can be obtained only under the condition that the optimum temperature distribution indicating the maximum characteristic of each part and the optimum temperature difference are added to both ends of the material. Therefore, in such thermoelectric materials and thermoelectric elements formed of these materials, the conventional performance evaluation method of measuring the above values at each temperature to evaluate the thermoelectric conversion performance index Z cannot be applied. .

[0006] Therefore, in the thermoelectric material and the thermoelectric element having the above-mentioned inclined structure, it is necessary to evaluate the basic performance of the thermoelectric element under the condition including the optimum design temperature difference and the temperature distribution. Therefore, the present invention is based on the following principle,
A method of evaluating the thermoelectric conversion efficiency and the average figure of merit under a certain temperature difference and temperature distribution condition was adopted.

That is, the parameters for evaluating the thermoelectric conversion efficiency include the maximum electric output (Pmax), that is, the internal resistance.
The thermoelectric conversion efficiency (η) when (Rm) is equal to the external resistance (Ro) (m = Rm / Ro = 1) is used. Now, as shown in FIG.
In the -N type thermoelectric element, the thermoelectric conversion efficiency (η) can be expressed by the following equation. η = Pmax / Qin (1) where Pmax = Vm ² / 4Rm (2) where Vm is the open-circuit voltage, Rm is the internal resistance of the thermoelectric element,
Qin is the amount of heat flowing into the thermoelectric element. In FIG. 5, Qout is the amount of heat flowing out of the thermoelectric element, and if there is no heat loss from the element to the surrounding environment, Qin and Qout show the same value. Further, the average thermoelectric conversion performance index can be determined from the relationship between the conversion efficiency (η) and the average thermoelectric conversion performance index (Z). Therefore, if the electric conductivity, the heat flux, and the thermoelectromotive force can be measured simultaneously, the conversion efficiency and the average figure of merit of the thermoelectric element can be evaluated.

However, the temperature at which the above-described graded composite thermoelectric material or element is actually used generally requires a temperature drop of several hundreds to 1000 ° C. from room temperature to high temperature. Under such a condition of high temperature head, there is a large temperature difference between the measurement sample and the surrounding environment, or between the high temperature part and the low temperature part of the measurement sample. The heat loss was large, and a one-dimensional heat flow environment from the high temperature part side to the low temperature part side of the thermoelectric element could not be obtained. Therefore, it is difficult to accurately measure the heat flow, and it is difficult to accurately evaluate the performance of the thermoelectric element and the thermoelectric material.

Accordingly, the present invention is to provide a method for accurately evaluating the performance of a thermoelectric conversion material or a thermoelectric element having a tilted structure, which has been difficult with the conventional method. Provided is a thermoelectric conversion element performance evaluation method and apparatus that can create a one-dimensional heat flow condition under the condition of a temperature drop and perform accurate performance evaluation under the actual use temperature conditions of the inclined structure thermoelectric conversion element and the conversion material. The purpose is to:

[0010]

In order to achieve the above object, a thermoelectric conversion performance evaluation method according to the present invention provides a method for evaluating the thermoelectric conversion performance of a thermoelectric element under a condition that causes a high temperature drop between one end and the other end of the thermoelectric element. In the thermoelectric conversion performance evaluation method for evaluating the conversion efficiency and average performance index, the surroundings of the thermoelectric
By keeping the temperature distribution in the axial direction of the heat insulating shield substantially equal to the temperature distribution in the axial direction of the thermoelectric element, the thermoelectric element maintains the one-dimensional heat flow condition. It was made.

Further, the thermoelectric conversion performance evaluation apparatus of the present invention
And disposed between the main heat source and the cooling source, the periphery of the thermoelectric device for measuring the like thermoelectric power, while surrounded by heat shields, the axial temperature distribution of the heat shields is the cooling and the main heat source An auxiliary heat source is provided for heating the outside of the heat insulating shield from the side so as to be substantially equal to an axial temperature distribution generated in the thermoelectric element due to a heat drop from the part. And in the thermoelectric conversion performance evaluation device,
It is desirable to fill the space between the thermoelectric element and the heat insulating shield with a filler having a low thermal emissivity and a low thermal conductivity because heat transfer can be prevented.

[0012]

In the thermoelectric conversion performance evaluation method of the present invention, in order to evaluate the thermoelectric performance, the temperature distribution of the environment around the thermoelectric element is measured under the condition that a high temperature drop occurs at both ends of the thermoelectric element to be measured. Is maintained substantially equal to the temperature distribution of the thermoelectric element. As a result, the heat flow that passes through the thermoelectric element from the high-temperature side to the low-temperature side maintains the one-dimensional heat flow condition necessary for measuring the heat flow to evaluate the performance of the thermoelectric element.

Further, according to the thermoelectric performance evaluation apparatus of the present invention, by heating the outside of the heat insulating shield from the side with the auxiliary heat source, a temperature distribution similar to the temperature distribution of the thermoelectric element is formed on the heat insulating shield. In addition, the one-dimensional heat flow conditions required when measuring the composite thermoelectric element having the inclined structure are realized with high accuracy.

Further, in the thermoelectric performance evaluation apparatus of the present invention, when a filler having a low thermal emissivity and a low thermal conductivity is filled between the thermoelectric element and the adiabatic shield, the thermoelectric element has a high temperature side to a low temperature side. The heat transfer due to the radiant heat toward is suppressed by the filler, and the one-dimensional heat flow condition is maintained with higher accuracy.

[0015]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of a thermoelectric performance evaluation device,
The measuring unit of the thermoelectric performance evaluation device 1 is installed inside a vacuum chamber 2 made of cylindrical quartz glass. The main heat source 3 composed of an infrared lamp is arranged at the upper part.
Is reflected by a condenser mirror provided above the main heat source 3 and is radiated downward.

On the other hand, below, the upper heat flow meter 5, the sample 6, and the lower heat flow meter 7 are arranged in a line in this order. Sample 6 is a thermoelectric element for evaluating performance, is formed in a cylindrical shape, and is provided with a PN electrode. The upper heat flow meter 5 and the lower heat flow meter 7 each have a
And is made of Inconel and copper.

The sample 6 is sandwiched between the upper heat flow meter 5 and the lower heat flow meter 7 and is fixed on the cooling block 11 by three alumina rods 10 pulled by a spring 9 with a force of about 3 kgf. Have been. Cooling water 13 is circulated inside the cooling block 11 serving as a cooling source, and cools the cooling block 11.

The periphery of the sample 6 is surrounded by a heat insulating shield 8, and a plurality of auxiliary heat sources 4 are arranged outside the heat insulating shield 8. Each auxiliary heat source 4
Is constituted by an infrared lamp, like the main heat source 3, and the outside of the heat insulating shield 8 is heated by a reflector whose height can be adjusted. The heat radiation amount of the auxiliary heat source 4 and the height of the reflection plate can be controlled from the outside. The auxiliary heat source 4 can control the temperature distribution in the axial direction of the heat insulating shield 8 so as to match the temperature distribution in the axial direction of the sample 6 caused by the temperature drop between the main heat source 3 and the cooling block 11. I have.

A main heat source is provided between each of the upper heat flow meter 5, the sample 6, and the lower heat flow meter 7 and the heat insulating shield 8.
In order to prevent heat transfer by radiation from the 3 side to the cooling block 11 side, an alumina bubbler 12 was filled as a filler having a low thermal emissivity and a low thermal conductivity.

FIG. 2 shows the thermoelectric performance evaluation device 1 shown in FIG.
FIG. 4 is a schematic diagram showing a structure around a sample 6 of FIG. As shown in the figure, a thermocouple 15 is attached to each of the upper heat flow meter 5 and the lower heat flow meter 7, and is connected to a thermocouple terminal board 14 (FIG. 1). The thermocouple terminal board 14 is connected to a measuring device (not shown) installed outside the thermoelectric performance evaluation device 1 by a thermocouple compensating lead wire.

As shown in FIG. 2, a nickel foil 16 is interposed between the upper heat flow meter 5 and the sample 6, and a silver foil 17 is interposed between the sample 6 and the lower heat flow meter 7. I have. In FIG. 1, these members are illustrated separately for the sake of explanation. However, in actuality, the members are in close contact with each other, and the cooling water is supplied from the high-temperature main heat source 3 as shown in FIG. The heat flow Q is generated toward the low-temperature cooling block 11 cooled by the cooling block 13. Further, a thermocouple is attached to an arbitrary position of the heat insulating shield 8 surrounding the sample 6, so that the temperature distribution of the heat insulating shield 8 can be measured.

In the present embodiment, the shape of the thermoelectric element to be evaluated is basically a cylindrical element with a PN electrode, but in the case of a rectangular element, it can be easily dealt with by changing the shape of the upper and lower heat flow meters. . Then, the maximum temperature of the high-temperature portion of the sample 6 is set to 1500K, and the temperature difference between both ends of the sample 6 is set to about 800K, and the temperature distribution, electric conductivity, heat flow rate, thermal electromotive force (open-end voltage) of the sample 6 and the like are set. Was measured. At this time, the temperature distribution in the axial direction of the heat insulation shield 8 is controlled so as to be the same as the temperature distribution of the sample 6, the upper heat flow meter 5, and the lower heat flow meter 7, and further, from the main heat source 3 side, the cooling block 11 To prevent heat transfer by radiant heat to the side,
An alumina bubbler 12 having a diameter of 1 mm to 2.8 mm was used.

Then, the Seebeck thermoelectromotive force and the output from each thermocouple 15 were taken into a computer via a 33-channel multiplexer amplifier and an A / D converter to perform data processing. In addition, when measuring the internal resistance of the sample, in order to eliminate the influence of the Seebeck thermoelectromotive force, a pulse current controlled by a computer was used, and the internal resistance was obtained from the average value of the voltage values at the time of the positive and negative currents. .

EXPERIMENTAL EXAMPLE Using the apparatus constructed as described above, a FeSi2P-N junction element having a diameter of 15 mm and a length of 16 mm was adopted as the sample 5 for the sample 5, and the performance was evaluated by performing the following measurements. went. The inside of the chamber in which the thermoelectric performance evaluation device 1 is installed is evacuated, the temperature is increased by the main heat source 3 and the auxiliary heat source 4, and the temperature of the upper heat flow meter 5 and the heat insulating shield 8 is controlled to be a predetermined temperature. . After reaching the thermal equilibrium state, the temperature distribution, Seebeck electromotive force, and electric resistance of the sample 6 were measured. Further, the magnitude of the heat flux was obtained from the temperature difference between both ends of the heat flow meter.

As a result, the temperatures of the high temperature part and the low temperature part of the device were 1076K and 442K, respectively, and a temperature drop of 634K was obtained. The Seebeck electromotive force and the internal resistance (specific resistance) of the element were 270 mV and 0.4, respectively.
It was 7 Ωcm.

Further, it is very important to realize a one-dimensional heat flow pattern under high temperature head conditions as in the apparatus of the present invention. A heat flow calibration experiment was performed using a standard sample made of (SUS304). As a result, when the temperature of the upper heat flow meter 5 was set to 1190K, the temperatures of the upper and lower portions of the standard sample were 898K and 787K, respectively.
K, and the magnitude of the heat flux through the upper heat flow meter 5 and the standard sample, respectively 21.6W / cm ² and 20.9W / cm ²
And there was a 3% attenuation. This result indicates that a good one-dimensional heat flow pattern can be obtained in the apparatus of the present embodiment, and the effectiveness of the method and apparatus for evaluating thermoelectric conversion performance according to the present invention has been confirmed.

[0027]

As described above, according to the thermoelectric performance evaluation method of the present invention, a temperature distribution similar to that of the thermoelectric element is formed around the thermoelectric element to be measured. Heat can be prevented from entering and exiting from the surrounding environment from the side of the thermoelectric element, and a one-dimensional heat flow condition can be realized inside the thermoelectric element. As a result, it is possible to evaluate the thermoelectric conversion performance of the composite thermoelectric element having the inclined structure under the actual use temperature condition with high accuracy.

Further, according to the thermoelectric performance evaluation device of the present invention, by heating the outside of the heat insulating shield from the side with the auxiliary heat source, the temperature distribution similar to the temperature distribution of the thermoelectric element measured on the heat insulating shield can be easily obtained. The one-dimensional heat flow condition required when measuring the composite thermoelectric element having the inclined structure can be realized with high accuracy.

Further, in the thermoelectric performance evaluation device of the present invention, when a filler having a low thermal emissivity and a low thermal conductivity is filled between the thermoelectric element and the heat insulating shield, the thermoelectric element has a higher temperature than a lower temperature. The transfer of heat due to the radiant heat toward the surface can be reduced, and the one-dimensional heat flow condition can be maintained with higher accuracy.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one embodiment of a thermoelectric performance evaluation device of the present invention.

FIG. 2 is a schematic diagram showing a structure around a sample of the thermoelectric performance evaluation device of the present invention.

FIG. 3 is a diagram showing the relationship between the temperature and the figure of merit of a conventional thermoelectric element.

FIG. 4A is a schematic view showing different material portions of a composite structure thermoelectric element having a tilted structure, and FIG. 4B is a view showing a relationship between the temperature and the figure of merit of the composite thermoelectric element having a tilted structure.

FIG. 5 is a schematic view of a PN type thermoelectric element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Thermoelectric performance evaluation apparatus 2 Vacuum chamber 3 Main heat source 4 Auxiliary heat source 5 Upper heat flow meter 6 Sample 7 Lower heat flow meter 8 Adiabatic shield 9 Spring 10 Alumina rod 11 Cooling water block 12 Alumina bubbler 13 Cooling water 14 Thermocouple terminal board 15 Thermocouple 16 Nickel foil 17 Silver foil

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tatsuo Kumagai 63-4, Higashi-Funaoka, larger section of the capital, Shibata-cho, Shibata-gun, Miyagi (56) References JP-A-60-39541 (JP, A) JP-A-5-95 18913 (JP, A) H. Woodbury, R .; S. Lewandowski, L .; M. Levinson and S.M. Lough in, in "Proceedings of the IX International Conference on Thermoelectrics (USA)", 1990, p. 303-322

Claims (3)

(57) [Claims] 1. A thermoelectric conversion performance evaluation method for evaluating a thermoelectric conversion efficiency and an average figure of merit of a thermoelectric conversion element under a condition that causes a high temperature drop between one end and the other end of the thermoelectric conversion element. Insulation around element
Surrounding the heat insulating shield so that the axial temperature distribution is maintained substantially equal to the axial temperature distribution of the thermoelectric conversion element, so that the thermoelectric conversion element maintains one-dimensional heat flow conditions. Characteristic thermoelectric conversion performance evaluation method. Wherein arranged between the main heat source and the cooling source, the periphery of the thermoelectric conversion element to make measurements such as thermal electromotive force, while surrounded by heat shields, axial <br/> of the heat shields temperature distribution, said the main heat source and heat drop of the cooling unit to be substantially equal to the axial temperature distribution generated in the thermoelectric conversion element, by providing an auxiliary heat source for heating the outside of the heat shields from the side The thermoelectric conversion performance evaluation device characterized by the above-mentioned. 3. The thermoelectric performance evaluation device according to claim 2, wherein a filler having a low thermal emissivity and a low thermal conductivity is filled between the thermoelectric conversion element and the heat insulating shield.